A conventional microcomputer system includes a microcomputer, a memory for storing a predetermined program connected to the microcomputer, and a timer (watch-dog-timer) connected to the microcomputer for detecting a runaway of a program in the microcomputer. The microcomputer carries out a program including a predetermined routine for checking running of the program. The timer times a running time of the microcomputer, before a timer clear command is conducted.
In operation, when a program runs on the check routine during normal operation, a clear signal is supplied to the timer from the microcomputer, so that the timer is reset. On the other hand, if the program does not pass on the check routine within a predetermined time, the timer supplies an overflow signal to the microcomputer which is thereby informed that a runaway of the program occurs therein. Therefore, a predetermined routine for removing the runaway is carried out in the microcomputer.
However, the conventional microcomputer system has a disadvantage in that there is a possibility that a runaway is not detected in a case where the timer is reset without conducting a predetermined program, and where operation falls in a non-limited loop including the conduct of the timer clear, so that the timer is reset each time in the loop.